1. Field of the Invention
This invention addresses the field of electronics. More particularly, this invention lies in the region of solid state devices. More specifically, this invention relates to variable resistance solid state devices that can effect a variable, linear attenuation of radio frequency (RF) signals. More exactly, this invention describes a circuit of a digitally controlled, exponential driving means for a logarithmic attenuating means to effect a linear attenuation of an RF signal. More specifically, but not necessarily limited thereto, this invention utilizes a companding digital to analog converter (DAC), which has characteristics of a digital input and exponential analog output, to drive, by means of a current source, a PIN diode. A PIN diode has characteristics of a variable resistor and logarithmic attenuator for attenuation of RF signals. The combination of the companding DAC and PIN diode effects a linear attenuation of an RF signal controlled by digital means.
Diodes, PIN diodes, and DAC's are well known and utilized in the art. The unique application of the invention disclosed herein requires a comprehensive understanding of diode characteristics.
Since the variable RF attenuation is dependent on the variable resistance of the PIN diode, it is worthwhile at this point to review the key properties of PIN diodes.
The most important feature of the PIN diode is its inherent ability to act as a current controlled resistor at Radio Frequencies. Most diodes possess this capability to some degree, but the PIN diode is especially optimized in design to achieve a wide resistance range with consistantly good logarithmic linearity and low distortion. In a typical application, as the control current is varied continuously from one microampere to 100 milliamperes, the resistance of a PIN diode will change from over ten thousand ohms to about one ohm. This characteristic variation of resistance with current makes the PIN diode ideally suited for attenuator applications.
The PIN diode is similar to ordinary P-N junction diodes except for an added intrinsic region (I-layer) sandwiched between the P and N layers.
The I-layer is merely a layer of ordinary semiconductor material (Silicon or Germanium), having all impurities removed. The nature of semiconductors is such that even very small amounts of certain impurities drastically alter their electrical properties. In an intrinsic semiconductor, the Si or Ge atoms each have four valence electrons in their outer orbits which conveniently form a covalent bond with four adjacent, similar atoms, leaving no free electrons for conduction of current through the lattice structure and hence indicating a strong resistance (resistivity).
The P type layer, is formed by adding a dopent impurity, such as B, Ga or In, having three valence electrons, to the intrinsic semiconductor. Said dopent also desires to have four valence electrons for covalent bonding, and in taking (accepting) one from an adjacent Si atom, it leaves a positive hole in the Si atom; i.e. the positive protons of the Si atom now out number the negative electrons by one. Said positive hole (a majority carrier) then migrates (conducts current) from one Si atom to another.
The N type layer is formed by adding a dopent impurity, such as Sb, As or P, having five valence electrons, to the intrinsic semiconductor. Said dopent also desires to have four valence electrons for covalent bonding, and in giving (donating) one to an adjacent Si atom, it creates a free negative electron (majority carrier). Said negative electron then migrates (conducts current) from one Si atom to another.
Thermal generation of additional free electrons and accompanying holes tends to cause minority carriers in the P and N layers; i.e. in the P layer holes are majority carriers and electrons minority carriers, and in the N layer electrons are majority carriers and holes minority carriers.
It is in the I layer that the control of minority carriers is enhanced. The resistance and large width of the intrinsic layer results in a high breakdown voltage and low capacitance. When a forward bias (negative terminal applied to the N layer and positive terminal to the P layer) is applied to the P and N layers, the injection of minority carriers into the intrinsic region increases the conductivity of the I layer. The forward bias forces negative electrons (majority carriers) from the N layer into the I layer (the space charge region of a normal P-N junction) which become minority carriers therein. The bias also forces positive holes (majority carriers) from the P layer into the I layer which also become minority carriers therein. The time in which said minority carriers take to combine indicate their lifetime. At RF, the minority carrier lifetime is long enough to conduct current, and depending on their concentration (bias) a variable resistance appears rather than rectification.
It should be apparent that a reverse bias will inject few if any minority carriers into the I layer and thereby indicates a substantial resistance barrier.
Above a limiting frequency the PIN diode acts as a pure resistance. This RF resistance is controlled by varying the forward bias which varies the amount of minority carriers in the I layer.
Below a limiting frequency, rectification occurs as in an ordinary PN diode. In the vicinity of the limiting frequency, there is some rectification in the resulting distortion. The amount of distortion is dependent on the bias current, the RF power, the frequency, and the minority carrier lifetimes.
To completely understand the application of a companding DAC to the present invention in combination with a PIN diode to drive said diode it is important to understand how the companding curve is generated.
Reference is hereby made to incorporate herein an article entitled Data Conversion With Companding DAC Devices published February 1978 by Advanced Micro Devices which gives a thorough in depth explanation of companding DAC's.
Generally, the companding curve output is a piecewise linear approximation of an exponential characteristic. Each segment of the curve is a chord and each segment or chord consists of several steps wherein the size of each step doubles as the chord number increases which in effect gives an exponential output current.
2. Description of the Prior Art
It is known that there are many applications for attenuator circuits. One example of such an application for such a circuit is a modulator wherein an RF input signal is selectively attenuated as a function of a control or modulating DC current to provide amplitude modulation.
It has been known that certain components such as for example, PIN diodes, can exhibit the properties of a variable resistor at microwave frequencies which are too high for rectification to take place because of the relatively large recovery time (minority carrier lifetime) of a fixed I (intrinsically doped) layer. As mentioned earlier, at zero or reverse bias, the I layer introduces a high resistance. Under forward bias, however, the injection and storage of carriers reduces the resistance of the I region.
Some prior art attenuators have been designed using this knowledge with such components as PIN diodes and field effect transistors to accomplish RF attenuation which is a function of the direct current (DC) bias current through the components. Such circuits have also frequently utilized a constant DC source to control the DC bias.
These previous applications of a PIN diode have described a "linear" attenuation of an RF signal in response to a DC control current that is substantially "linear" over a wide dynamic range. The linear feature in all prior art, however, in fact refers to a logarithmic linearity in response to a linear DC control current. In fact the primary performance problem of PIN diode attenuators has been variable-gain nonlinearity.
In addition, all prior art dealing with an attenuation of an RF signal describes an analog input/output control. Today, we live in a digital world. Modern electronic systems are replacing many of the analog signal processing and transmissions functions with digital data processing. The use of digital electronics can lead to improvements in system cost, performance, accuracy and reliability. Digital systems can transmit many signals on the same line in a multiplexed mode and do not suffer from the same kind of noise and crosstalk problems that are inherent in analog systems.
Therefore it is substantially more desirable to be able to control RF attenuation linearly by digital means than by analog means as was customary in the past.
Prior to the invention disclosed herein one could with substantially more components, equipment and expense control an RF attenuation by digital means, as follows: on receiving a digital input control signal, an "exponential look up table" would be referenced to determine a respective analog voltage from a DAC that would cause a specific resistance (attenuation) in the PIN diode. This system though effective for its intended purposes was cumbersome, costly and extensive in the use of components and time.
Thus, there is a continuing need in the state of the art for an uncomplicated linear attenuator circuit that is digitally controlled.